Page:Popular Science Monthly Volume 60.djvu/404

396 conditions so that the whole plant is dwarfed. The above remarks apply to a frond or rather to the midrib of a palm when looked at in cross-section. When considered in cross-section the plumose fronds vary in a striking manner. The leaflets are arranged upon the midrib so that they may be in two rows or in four or six or more rows—half on one

side and half on the opposite side of the petiole. Seen in cross-section these latter take on one of the forms shown in the following diagram or one of many other combinations. These differences between fronds depend to a great extent on the manner in which the leaflets are attached to the midrib. For example, the leaflets may lie in a single plane growing straight out from the opposite sides of the midrib or they may lie in two, four, six or eight planes that meet along the midrib.

The fronds have, in addition to these peculiarities, certain habits due to the shapes of the midrib. A midrib that is broad at the base and continues relatively broad to the end is compelled to remain, as to cross-section, in a horizontal position; but if a midrib is broad at the base and gets rapidly narrower toward the end it cannot maintain itself in a horizontal position, but twists a fourth of the way round and at the end lies on edge. Sometimes this twisting goes to such an extent that the frond is quite inverted. The cross-sections of the midribs of palm fronds are characteristics to which but little attention seems to have been given by botanists.

Every palm leaf begins its life at the apex of the trunk—the newest